In many industrial processes using a gaseous feed stream it is desirable or necessary to remove carbon dioxide from the gaseous feed stream prior to certain steps of the process. For example, in the separation of atmospheric air into its component parts by cryogenic distillation; it is necessary to prepurify the air by removing carbon dioxide and water vapor from the air feed prior to refrigerating the air; otherwise, these gases would condense and freeze in the refrigeration heat exchange equipment and eventually clog the equipment, thereby necessitating removal of the equipment from service for removal of the frozen carbon dioxide and ice. The carbon dioxide and water vapor can be removed from the air by a number of techniques.
One well known method of removing carbon dioxide and water vapor from gas streams is by the use of pairs of reversing heat exchangers that are operated alternately, such that one heat exchanger is in purification service while the other is undergoing frozen carbon dioxide and ice removal. Specifically, in this method the gas feed is passed through one heat exchanger in exchange with a refrigerant, which causes the carbon dioxide and water vapor to freeze onto the surfaces of the heat exchanger. When the buildup of frozen carbon dioxide and ice in the heat exchanger reaches a certain level, the heat exchanger is taken out of service to remove, by sublimation and melting, the frozen carbon dioxide and ice. The other heat exchanger of the pair, from which frozen carbon dioxide and ice have been removed, is then placed into purification service. This method has the disadvantage that a considerable amount of purge gas is required to remove the frozen carbon dioxide and ice.
A popular method of removing carbon dioxide and water vapor from gas streams is adsorption. One common adsorption method used for air prepurification is PSA using two serially-connected adsorption layers, the first layer containing a desiccant, such as silica gel or activated alumina for water vapor removal, and the second layer containing a carbon dioxide-selective adsorbent, such as sodium-exchanged type X zeolite (13X zeolite). Typical two-layer air prepurification PSA processes are described in U.S. Pat. Nos. 5,110,569 and 5,156,657, the disclosures of which are incorporated herein by reference. This method has a number of disadvantages. It is difficult to desorb carbon dioxide from the 13X zeolite. The zeolite develops "cold spots" in the upstream region of the layer of zeolite adsorbent and the adsorbent loses some of its adsorption capacity with time. TSA has also been practiced using this combination of layers. U.S. Pat. No. 5,110,569, mentioned above, shows such a process. A major disadvantage of the described TSA process is that a great quantity of heat energy is required in the adsorbent regeneration step, since both layers must be heated sufficiently to drive off the adsorbed moisture and carbon dioxide.
Air prepurification by PSA has also been practiced using a single bed of adsorbent which removes both water vapor and carbon dioxide. Such a process is disclosed in U.S. Pat. No. 5,232,474, the disclosure of which is incorporated herein by reference. The principal disadvantages of this method of air prepurification are that it is difficult to efficiently produce high purity air (air containing less than 1 ppm carbon dioxide) by this method, and a high volume of purge gas is required to effect adequate adsorbent regeneration
Japanese Patent Publication No. Sho 55-27034 discloses an air purification process in which moisture and carbon dioxide are removed from air by a combination of PSA and TSA in an adsorption system comprising three adsorption vessels arranged in parallel. Each of the vessels contains a synthetic zeolite adsorbent. At any given time during the process two of the adsorption vessels are operated in an alternating PSA cycle while the adsorbent in the third vessel is thermally regenerated. Following regeneration of the adsorbent in the third vessel one of the other two is taken out of PSA service and thermally regenerated and the freshly regenerated vessel is put into PSA service with the other vessel that was in PSA service. This procedure is repeated continuously throughout the air purification process.
Methods of producing air containing very low levels of water vapor and carbon dioxide are continuously sought. The present invention provides a method which accomplishes this, and does so with low energy and capital expenditures